A systematic international effort
to map the entire global ocean floor to at least 100 meters spatial
resolution is a worthwhile and feasible project to begin the
new millennium, say scientists from the Naval Research Laboratory's
Marine Geosciences Division. Dr. Peter Vogt and his colleagues
laid out the research issues of such a project at the 1999 American
Geophysical Union (AGU) meetings in Boston, MA, and further developed
the concept in the June 6 2000 issue of the AGU publication,
Eos. Informal dialogues with academic and other government
scientists have consolidated thoughts on the concept, dubbed
GOMaP (Global Ocean-floor Mapping Project). Open public access
to all GOMaP data would be an essential ingredient for this
project.
The "resolution" of
an image, whether a photo or a geophysical map, is its sharpness.
In an image with a spatial resolution of 100m, objects of about
this diameter or width are barely detectible. Dr. Vogt and his
collaborators note that most or all of the Moon, Mars, Venus,
and several of the Jovian moons have been imaged to 100m or better
resolution in recent years, while our own planet's ocean floor
continues to be mapped piecemeal by many institutions for many
different purposes, using a variety of instruments of various
accuracies and resolutions. Only a few percent of the sea floor
has been mapped -- by acoustic backscatter imagery (sidescan
sonar) and swath bathymetry (echo-sounding) -- to the 100m resolution
returned from our Moon by the NRL-engineered Clementine mission.
Even this small percentage forms an inhomogeneous crazy-quilt
coverage concentrated near industrialized nations and a few sections
of the Mid-Oceanic Ridge.
Sea floor imaging in the visual
spectrum -- the basis for our new views of the Moon and Mars
-- can be achieved from above only in very shallow, clear water.
Several operational systems can map sea floor topography from
aircraft, using scanning lasers. NRL's Remote Sensing Division
has demonstrated shallow water sea floor imaging by hyperspectral
scanning, using passive illumination by sunlight. In most of
the oceans, where the water depths are generally 3,000 to 6,000
meters, visual-spectrum imaging can only be done from manned
submersibles or unmanned, remotely operated or towed instruments
deployed close to the ocean bottom. High frequency sonars can
also be deployed on such platforms, complementing the visual-spectrum
techniques.
To date, the best homogeneous
image of the global ocean floor is actually the ocean
surface shape, called the geoid, which largely reflects
sea floor topography. The ocean geoid has been mapped using satellite
radar altimetry, as first proposed by NRL's Dr. Benjamin Yaplee
in 1969. Although of great scientific value as demonstrated
by NRL and other geoscientists this sea floor "image"
is some 100 times poorer in spatial resolution than images returned
from the Moon by the Clementine mission.
Dr. Vogt and his colleagues at
NRL's Marine Geosciences Division and elsewhere had been studying
the Norwegian-Greenland Sea geology and geophysics for more than
a decade using 2 to 20 km-resolution techniques. While much was
learned at these spatial scales, only subsequent 10 to 100m (and
better) resolution sea floor acoustic imagery collected by NRL
revealed the wealth of previously unknown features and processes,
including an active sea floor mud volcano. Similar revelations
by other scientists in other oceans only hint at the new discoveries
that GOMaP could return.
Imaging the entire global ocean floor at 100m spatial resolution
is no mean feat; at about 370 million square km, the area of
our planet's water-covered lands exceeds the area of Mars plus
three Moons. However, the task can be done now with state-of-art
surface-ship-based technology, or perhaps, in some areas, by
small fleets of autonomous underwater vehicles at higher resolution,
with appropriate technological advances in the near future. However,
platforms move through the water at speeds of only meters per
second, not kilometers per second like orbiting spacecraft.
Although only a small percentage
of the ocean floor lies at very shallow depths (less than 50m),
the narrow sonar swath widths in shallow water translates into
large amounts of ship time. However, the resolution of imagery
is progressively better as water shoals , and may be better than
1m in waters less than 50m deep.
With support from the Office
of Naval Research (ONR), NRL hosted a small pilot workshop in
Bay St. Louis, Mississippi, June 12-14 2000. A small group of
experts from academia, the commercial sector and U.S. government
agencies assembled to examine critically the technological and
data-handling/data-analysis issues of GOMaP. The workshop considered
the Juan de Fuca tectonic plate (off Washington and Oregon) and
a large part of the Gulf of Mexico as "pilot" areas
where GOMaP technology can be demonstrated and evaluated. Attendees
agreed that GOMaP was feasible and of great potential value for
mankind. A larger, more international conference will be planned
for 2001.
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 85 years and continues to meet the complex technological challenges of today's world. For more information, visit the NRL homepage or join the conversation on Twitter, Facebook, and YouTube.
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